1. Field of the Invention
The present invention relates to a sigma-delta modulator, and more particularly, to a sigma-delta modulator comprising a continuous-time integrator.
2. Description of the Related Art
In general, a sigma-delta modulator has many advantageous features. For example, the sigma-delta modulator often has a high resolution and a high signal-to-noise ratio (SNR). The sigma-delta modulator is also a simple circuit, which doesn't need additional trimming circuits to perform its function. It is not sensitive to clock jitter, and has a good endurance against the variances of the circuit components. Please refer to FIG. 1, which is a diagram of a sigma-delta modulator 100 according to the prior art. As shown in FIG. 1, the sigma-delta modulator 100 is a discrete-time 2-stage (or more stages as indicated by the series of dots between the first discrete-time integrator 102 and the nth discrete-time integrator 104) sigma-delta modulator. The digital signal Y(z) outputted from the analog-to-digital converter (ADC) 106 is transmitted to the input end of each 1-stage discrete-time integrator 102, 104 as a feedback through the digital-to-analog converter (DAC) 108. It can be easily seen that the above-mentioned structure utilizes a feedback mechanism. Therefore, for an m-stage sigma-delta modulator, as the value of the number m increases, the resolution and the SNR of the sigma-delta modulator will be better. Furthermore, different resolutions and the SNRs can be derived through adjusting the coefficient of the inner integrator.
However, it is well-known that the above-mentioned discrete-time sigma-delta modulator 100 has some disadvantages. For example, it may encounter a signal aliasing phenomenon. The signal aliasing phenomenon arises when the discrete-time sigma-delta modulator 100 receives signals having different tones. When some of the sampling rates of the received signals are multiples of sampling rates of other received signals, the curves corresponding to all the received signals on the spectral plot overlap. The signal aliasing phenomenon makes it difficult to distinguish the signals having different tones.
In order to overcome the signal aliasing phenomenon, an anti-aliasing circuit should be established in the signal-receiving end of the sigma-delta modulator. However, setting the anti-aliasing circuit makes the entire discrete-time sigma-delta modulator 100 more complex.
Another solution is to utilize a continuous-time integrator as a first-stage integrator of the sigma-delta modulator 100. In this way, the anti-aliasing circuit can be simplified. However, a time delay of the signal is introduced due to the continuous-time integrator and the SNR of the sigma-delta modulator 100 is reduced.
Therefore, it is apparent that new and improved methods and devices are needed.